How many blocks do I need?
Concrete blocks are counted the same way as bricks: divide the wall area by the face area of one block plus its mortar joint. The standard US concrete masonry unit (CMU) is nominally 8 × 8 × 16 in — its actual size is 7⅝ × 7⅝ × 15⅝ in so that, with a ⅜ in joint, each block covers exactly 16 × 8 = 128 in².
blocks needed = net wall area × blocks per area × (1 + waste %)
So a 40 × 8 ft wall (320 ft²) needs 320 × 1.125 = 360 blocks, or 378 with 5% waste. A metric 390 × 190 mm block with 10 mm joints covers 0.4 × 0.2 = 0.08 m², so you need 12.5 blocks per m²; a UK 440 × 215 mm block needs about 9.9 per m².
| Block | Actual face size | Per ft² | Per m² | Per 100 ft² |
|---|---|---|---|---|
| US CMU 8×8×16 (also 4", 6", 10", 12" wide) | 15⅝ × 7⅝ in | 1.125 | 12.1 | 113 |
| Metric hollow block 390 × 190 | 390 × 190 mm | 1.16 | 12.5 | 116 |
| UK block 440 × 215 | 440 × 215 mm | 0.92 | 9.9 | 92 |
The width of the block (4, 6, 8, 10 or 12 in) doesn't change the count — only the face size does. Width does change the weight, the strength and the amount of grout if you fill the cores.
Mortar for a block wall
Hollow concrete blocks are normally laid with face-shell bedding: mortar goes only on the two face shells, not across the webs. The common estimating figure is about 8.5 ft³ of mortar per 100 blocks (8×8×16, ⅜ in joints), and the calculator uses that rate (about 0.029 m³ per m² of wall) for hollow blocks. Solid blocks, such as UK dense or aircrete blocks, are calculated from their full joint volume. Add a mortar waste allowance on top — 15% is the default.
Block mortar is usually a little stronger than brick mortar. A 1 : 4 cement–sand mix, or US Type S or N mortar, is typical. For reinforced walls, the cores holding rebar are filled with grout, which isn't included here.
Concrete block vs cinder block
"Cinder block" is the everyday name for any hollow masonry block. True cinder blocks were made with coal cinders (fly ash) as aggregate and are lighter but weaker; nearly all blocks sold today are concrete blocks made with sand and gravel or lightweight aggregate. For estimating, they're the same — use the block's face size.
Tips for estimating a block wall
- Corners and ends. Corner and bullnose blocks replace standard blocks one for one, so the count stays the same — just tell the supplier how many of each you need.
- Openings. Add doors and windows in step 2 to subtract them. Remember lintels and bond-beam blocks over openings.
- Half blocks. Running bond needs a half block at the end of every other course on free ends. Waste of 5% usually covers this, or you can cut full blocks.
- Retaining walls built from mortarless segmental blocks have their own retaining wall block calculator.
Frequently asked questions
How many concrete blocks are in a square foot?
1.125 standard 8×8×16 blocks per square foot, or 112.5 per 100 square feet, with ⅜ in joints. 4", 6", 10" and 12" wide blocks have the same 8 × 16 face, so the count is the same.
How many blocks are in a square meter?
About 12.5 metric 390 × 190 mm blocks per square meter with 10 mm joints, or about 10 UK 440 × 215 mm blocks.
How many blocks do I need for a 10 ft wall?
It depends on the height. A 10 ft long × 8 ft high wall is 80 ft², which needs 90 blocks (8×8×16), or about 95 with 5% waste.
How much mortar do I need for 100 blocks?
About 8.5 ft³ of wet mortar for 100 standard 8×8×16 blocks laid with face-shell bedding, before waste. Full bedding uses more.
Should I add waste for blocks?
Yes — 5% is usual for blocks, which break less than bricks but still need cutting at ends, corners and openings.